Using a laser bar code reader or scanner for reading a bar code, that is, decoding the encoded indicia of a target bar code represented by elements of the bar code, is well known. The laser scanner generates a beam of light, typically a laser beam, which is repeatedly scanned across a target bar code. The elements or features of a bar code, e.g., black bars and white spaces of a UPC bar code, absorb and diffusely reflect the laser beam light. Reflected light from the bar code is collected and focused on one or more photodetectors of the laser scanner. Output signals from the one or more photodetectors are appropriately processed and then input to decoding circuitry of the scanner and decoded.
To collect and direct reflected light onto a photodetector, optical focusing lenses have been employed in laser scanners. Optical focusing lenses however generally require larger photodetectors and, thus, such systems may be more costly. Alternately, non-imaging light collectors have been utilized in laser scanners to facilitate reading of bar codes. For example, U.S. Pat. No. 5,357,101 to Plesko discloses a non-imaging light collector used to collect and concentrate light on a photodetector of a laser scanner. The Plesko '101 patent is incorporated herein in its entirety by reference.
Non-imaging light collectors have an advantage over conventional lenses for non retro-reflective light collection systems in laser scanners because they can provide a wide field of view while concentrating collected light onto a small photodetector region. Wide fields of view can be accommodated by conventional lens systems, but the required size of the photodetector is increased. Smaller photodetectors are advantageous in that they exhibit lower capacitance and minimize noise thereby increasing the signal-to-noise ratio.
In the past, laser scanners utilizing non-imaging light collectors have been limited to working at relatively short ranges, typically, limited to no more than around 20 inches from the scanning head to the target bar code. Scanners with this type of short range typically require relatively wide scan angles of around 50 degrees to successfully read bar codes, thus, non-imaging light collectors provide a good solution for short working range laser scanners with wide scan angles. However, some scanning applications require longer working range, up to several feet. These longer range scanners operate at much narrower scan angles, typically 10 to 15 degrees. However, the greater the distance between the scanner and the target bar code, the more difficult it is to obtain sufficient reflected light from the target bar code such that the photodetector output signal can be successfully decoded. Moreover, the greater the distance between the scanner and the target bar code, more sources of ambient light, which constitutes noise to the scanner decoding process, have to be dealt with. Thus, longer range scanners require collection of as much reflected laser light from the target bar code as possible while minimizing ambient light received by the photodetector.
What is needed is a light collection system for a laser scanner that provides for extended working range laser scanning while minimizing required photodetector size.